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heat treatment electric furnace
QUESTION:Could anyone tell me about building an electric furnace for steel
treatment?
ANSWER: I found an old dental oven about 12" x 12" x 12". It came with an
analog temperature meter (like a volt meter) and had a broken heating
element. I replaced the element with one salvaged from an old clothes
dryer. That did not last long so I bought a new clothes dryer heating
element, replaced the element again, and that one is still in use
(used elements get pretty brittle and I stressed it in re bending it
to fit).
Mine runs off of a 25A 220V (1 PH) outlet for the heating element and
it will get up to and over 2000 degrees from cold in less than 5
minutes.
I found a temperature controller that uses type "J" thermocouples at a
surplus place (this was before eBay). The input to the controller was
to simply connect two wires (red and yellow) across the analog meter.
The controller reads the voltage from the thermocouple and opens and
closes an internal (to the controller) relay as the voltage rises and
falls.
I was amazed to note that I always got very similar temperature
readings from the old analog meter that came with the oven and the one
on the controller. I could always tell what the air temperature was
in my shop (if the oven had not been used) by checking either of the
analog meters.
The controller runs off of a 110V outlet. The internal relay will
handle 110V and several Amp and is accessed via a contact strip. I
use that to switch a large double pole relay (another trip to the
surplus place) that opens and closes the hot legs to the heating
element (turns the oven on and off). The big relay is rated at about
50A on the switched side.
My controller has an analog dial (looks like old analog volt meter
dial) with an adjustable pointer for setting the temp. In use, I plug
in the oven and the controller, I set the temp at the temperature
wanted, and turn the controller on. The oven relay closes, and the
temp starts climbing. Once it reaches the set temp, it floats (off at
set and on at about -3 or - 5 degrees from the set temp if I recall
right) and the relay opens and closes accordingly. It probably cycles
on and off every 15 or 20 seconds in a garage on a warm California
day.
I like hearing the relay clicking because it reminds me that I have a
volatile task in progress.
I used the oven more for tempering than for hardening. I was more
comfortable with using propane or O/A torches to bring the parts up to
the hardening temp and then quenching them to harden them.
Trying to work in through an open oven door and opening and closing
the door and trying to judge colors or use a magnet to check for
hardening temp was awkward and the heat made it very uncomfortable.
So I usually closed all the doors, turned most of the lights out and
hardened and quenched "in hand" (actually, I used wraps of SS safety
wire, welding gloves, and pliers to hold the parts usually).
For tempering, I would set the temp needed, bring the oven up to that
temp and let it stabilize, and lay the part on top of half a fire
brick that stood on end in the center of the oven.
I used Brownells times and temps for drawing (that word is synonymous
with tempering), it was usually one hour at 350 to 550 degrees. At
the end of that time I would shut the controller off and let the part
come to room temp in the oven. That could take two hours.
I was using O-1 and W-1 mostly, small making gun parts, and had
excellent success by simply following the instruction in the Brownells
(http://www.brownells.com) catalog. I also made a couple of special
cutters for making gun parts with. I bought most of my tool steel
from Brownells, I was a low volume user and they are pleasant to deal
with and very reliable and efficient. And they sell in smaller
quantities. Their catalog is worth its cost ($5?) just for reference
purposes.
My oven is lined with that soft, sort of yellowish white fire brick,
it has rectangular grooves in the sides and back that are sized to
hold the element just about flush with the brick faces. The element
is held in place by little "U" shaped wire clips that are pressinto
the bricks.
My question? What is that type of brick called? And is it still
available? And what is the cement used for mounting and repairing the
bricks? My oven is in storage but I will give it a little overhaul
when I get a place to use it again. It is probably 50 or more old.
Brownells sell some heat treating ovens too, I think then are priced
at retail for the most part.
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